1. Field
The present invention pertains to low inductance resistors, and in particular, to resistors designed for use in the emitter circuits of Class A RF power transistors.
2. Prior Art
A primary concern in the design of Class A RF power transistor circuitry is thermal runaway. During thermal runaway, the transistor current continuously increases until the transistor thermal dissipation limit has been exceeded. Some form of dc stabilization circuitry is usually necessary to prevent thermal runaway and maintain the transistors at a suitable, stable operating point, despite changes in the dissipation and ambient temperature.
Stabilization circuitry for RF power transistors commonly includes either collector or emitter feedback. A circuit employing collector feedback is shown in FIG. 3. This circuit comprises a trnsistor 1, a resistor 7, and a feedback network 8. In the operation of the circuit of FIG. 3, an increase in collector current is sensed across the resistor 7 by the feedback network 8. The feedback network adjusts the transistor bias supply accordingly to reduce, and thereby stabilize, the transistor collector current. The feedback circuit usually includes one or more transistors to amplify the voltage sensed across the resistor 7 and provide a means of accommodating the difference in voltage between the collector and base circuits.
Although the collector feedback circuit does stabilize the collector current, there are several disadvantages inherent in its operation. For example, the additional transistors in the feedback circuit increase the size and cost of the overall amplifier. The feedback network includes a delay which may result in the failure of the RF power transistor because the feedback reaction is not quick enough to protect the RF transistor. The feedback network reduces collector current caused by high signal levels and results in undesired signal compression. Finally, the feedback circuit reduces reliability because it includes a substantial number of additional active components. The reliability problem is further complicated because the failure of a relatively low cost component, such as a transistor in the feedback circuit, can result in the failure of the more costly RF power transistor it was intended to protect.
Emitter feedback circuits overcome most of the disadvantages of collector feedback circuits, however, the emitter feedback circuits present a number of different problems. A circuit employing emitter feedback is shown in FIG. 1. This circuit comprises a transistor 1, an emitter feedback resistor 3, and an emitter bypass capacitance 4. In the operation of this circuit, an increase in emitter current increases the voltage across the emitter resistor 3. This voltage tends to reduce the forward bias across the emitter-base of the transistor and thereby operates to stabilize the transistor current.
Unless an emitter resistor is bypassed for RF current, it reduces the RF gain of the RF power transistor. Where a relatively high value of emitter resistance is employed, a low impedance capacitor placed in shunt effectively bypasses the emitter resistor, leaving the RF gain of the transistor relatively unaffected. However, where a low value emitter resistance is employed, for example one ohm, it is difficult to provide a bypass capacitor which has a sufficiently low impedance to be effective as a bypassing element. In addition, the impedance of the emitter resistor and any bypass capacitor may well exceed one ohm at RF frequencies, despite their nominal value, because of lead inductance or device inductance caused by physical size. High power application may require the bypass capacitor to carry current as high as ten amperes. A low value of equivalent series resistance in the capacitor can produce a significant voltage drop. Finally, the bypass circuit can have parasitic elements which cause it to parallel resonate and destroy the gain of the amplifier at a frequency in the passband.
FIG. 2 shows a conventional emitter resistor and bypass capacitor drawn approximately to scale. It can be seen that there is appreciable lead length associated with both the resistor and the capacitor. The lead length inductance and losses associated with the capacitor usually make bypassing for low values of resistance impractical.
Where a low value of emitter resistor is chosen, the resistor is generally employed without bypassing, and an effort is made to reduce lead length inductance and other inductances associated with the resistor. One method of reducing the resistor inductance is to parallel a number of resistors. Unfortunately, this approach produces a bulky assembly and the reduction in inductance obtained is usually far from optimum. A number of modern power transistors employ internal emitter resistors to balance each of the individual transistor elements that make up the overall transistor; however, these resistances are insufficient for Class A biasing and external resistance is generally required.
Thin film resistors of the type shown in FIG. 4 have a lower inductance than conventional resistors. The resistor of FIG. 4 comprises a resistive element 9, an input contact 10, and an output contact 11. Current flows from the input contact through the resistance element to the output contact in a longitudinal direction, as shown by the arrow 12. The relatively short lead length tends to reduce the inductance of this type resistor over conventional carbon resistors, but the inductance is still appreciable because of the length of the body of the thin film resistor and its relatively narrow cross sectional area.